This invention relates to a process for the protection against corrosion of the steel reinforcement in reinforced concrete and to novel reinforced concretes having improved resistance to corrosion.
Steel reinforcement in concrete is normally protected against corrosion by a passive film that forms on its surface in the alkaline environment in the concrete. However, with the passage of time alkalinity may be lost by the action of atmospheric carbon dioxide and corrosion may result either from this loss of alkalinity or from the contamination of the concrete with aggressive ions such as chloride. Both these processes render the protective passive film unstable.
When the concrete is highly alkaline it tolerates a small level of chloride ions without corrosion of the steel being initiated. However the higher the chloride content the higher is the risk of chloride induced corrosion. The chloride content that results in corrosion initiation is termed the chloride threshold level. The initiation of corrosion may be detected electrically and is marked by a sharp increase in electrical current. It has been reported that chloride contents above 0.2% by weight of cement will initiate corrosion in many reinforced concrete structures.
It has therefore been previously proposed to remove chloride by an electrochemical process. The process has been described in WO 98/35922, and European Patent Nos 200,428 and 398,117 and involves passing an electric current through the concrete by applying a voltage for example from 3 to 15 volts between an external temporary anode and the steel reinforcement in the concrete as cathode. The effect is to cause chloride ions to migrate through the concrete to the surface and into a layer of electrolyte which has been placed at the surface.
It has been previously reported that a factor affecting corrosion initiation in chloride contaminated concrete is entrapped air voids. Concrete typically contains about 1.5% by volume of entrapped air. The presence of voids at the steel surface increases the risk of the local environment being altered by the presence of chloride ions to generate conditions in which the passive film is unstable. The solid hydration products of cement, which are absent at these locations. would otherwise have corrosion inhibiting properties that resist such changes.
The effects of loss of alkalinity by the action of carbon dioxide in the atmosphere. chloride contamination and the presence of voids in the concrete mean that with the passage of time the steel reinforcement becomes susceptible to corrosion. The present invention provides a means of reducing this problem in which the resistance of the steel to corrosion is increased by controlling the amount of air voids in the concrete and providing a layer of solid alkali on the steel surface.
According to the present invention there is provided a reinforced concrete wherein the content of voids in the concrete at the surface of the steel reinforcement is below 0.8%, preferably below 0.5%, more preferably below 0.2% by area of steel and in which there is a layer of solid alkali on the steel surface.
The provision of the layer of alkali and the low void content has the effect of inhibiting corrosion. The combination raises the chloride threshold for chloride induced corrosion from levels in the region of 0.2% chloride by weight of cement to above 0.5% and even above 1.5% or 2%. This greatly enhances the durability of reinforced concrete.